Ah I see now, having read more of the history. Calling it that seems pretty misleading/odd.
(We use GitLab and Mattermost (integration) where I work. I've been 'remote / WFH' for the past 7 years.)
I agree it can be improved and created https://gitlab.com/gitlab-org/omnibus-gitlab/-/merge_request... to do so.
https://about.gitlab.com/blog/2015/08/18/gitlab-loves-matter...
> Like many companies in the last year we've switched to using Slack to improve internal communication. [...] Since Slack doesn't offer an on-premises version, we searched for other options. We found Mattermost to be the leading open source Slack-alternative and suggested a collaboration to the Mattermost team.
I'm not really sure why it's 'GitLab Mattermost' and not (at your link) 'GitLab Nginx' et al. though.